The present invention relates to a phase compensation circuit for compensating various characteristics of an automatic control system.
For instance, in a servo system, a phase lead circuit and a phase lag circuit are frequently used as a cascade compensating element. FIG. 1 is a circuit diagram showing a typical phase lead circuit. As is well known, a transfer function T(S) of the phase lead circuit is expressed as follows: ##EQU1## Moreover, an amplitude-frequency function A(.omega.) and a phase-frequency function .phi.(.omega.) are respectively represented by the following equations: ##EQU2## FIGS. 2A and 2B are graphs showing characteristics of the amplitude-frequency function A(.omega.) and phase-frequency function .phi.(.omega.), respectively. In the graphs shown in FIGS. 2A and 2B, a center frequency .omega..sub.0 at which the maximum phase angle is obtained can be expressed as follows: ##EQU3## and an amount of the maximum phase angle .phi..sub.0 at the center frequency .omega..sub.0 is represented by the following equation (5): ##EQU4##
The center frequency .omega..sub.0 and the maximum phase angle .phi..sub.0 mentioned above can be determined by suitably selecting values of resistors R.sub.1, R.sub.2 and a capacitor C, provided that a frequency characteristic of an actuator, an error signal detection sensitivity, a servo loop gain, etc. of the servo system are all fixed. However, when the servo gain is to be adjusted or the frequency characteristic of the actuator is varied due to a temperature dependency thereof, it is extremely difficult to set up again the phase compensation characteristic in the most suitable condition in response to such an adjustment or variation. This is because of the fact that in the known phase compensation circuit, the phase-frequency characteristic could not be directly adjusted regardless of whether it is an active type or a passive type, but is varied in an indirect manner by changing cut-off frequencies .omega..sub.1 and .omega..sub.2 of the amplitude-frequency characteristic. Further, in the known phase compensation circuit, it is impossible to directly adjust the center frequency .omega..sub.0 and the maximum phase angle .phi..sub.0 independently from each other. In other words, if the center frequency .omega..sub.0 is adjusted to a desired value, the maximum phase angle .phi..sub.0 might be also changed.